1. Field of the Invention
The present invention relates to a liquid crystal display device having low dependency on the viewing angle, and good visibility independent of the viewing direction.
2. Description of the Related Art
TN mode liquid crystal display devices generally have the problem of dependency on the viewing angle. FIG. 10 shows the general dependency on the viewing angle of a TN mode liquid crystal display device, in which an area surrounded by a curve represents a contrast (CR) range of 10 or more. This figure indicates that the TN mode liquid crystal display device has somewhat good visibility in the transverse direction, but has poor visibility in the upward or downward direction.
On this background, various structures have recently been proposed for widening the viewing angle of a liquid crystal display device.
One of such structures is a divided-alignment structure on the pixel unit. The divided-alignment structure is a structure having domains in which liquid crystal molecules are raised in different directions by applying a voltage to each of pixels. For example, each pixel is divided into two regions, and the alignment films of the two divided pixel regions are subjected to alignment in different directions to realize this structure. In this divided alignment structure, an abrupt asymmetric contrast change in the vertical direction, which is the problem of TN mode liquid crystal display devices, is decreased and made symmetric, thereby obtaining the effect of widening the gray-scale area without reversal of gray scales. This leads to realization of a liquid crystal display device having a wide viewing angle.
FIG. 9 shows an example of the method of manufacturing a liquid crystal display device having the in-pixel divided alignment structure, and particularly shows the procedure for alignment processing. For example, the substrate 1 shown in FIG. 9A comprises source wiring (not shown) and gate wiring (not shown), which are disposed in a matrix form, and a pixel electrode 2 made of a transparent conductive material such as ITO or the like and provided in each of the regions surrounded by the source wiring and gate wiring, and a thin film transistor (not shown) connected as a switching element to each of the pixel electrodes 2. The surfaces of the pixel electrodes 2 are coated with an alignment film 3 made of a material such as polyimide or the like. On the substrate 1 are further provided a black matrix 4 coated on the non-display region around the pixel electrodes 2, and an insulating film 5.
The substrate 1 is rubbed in the direction shown by an arrow A in FIG. 9A. This rubbing can be carried out by rubbing the surface of the alignment film 3 with a roller having a rubbing cloth wound thereon.
Next, a photoresist film 6 is deposited to cover a half region of each pixel, as shown in FIG. 9B, and then rubbed in the direction shown by an arrow B opposite to the arrow A shown in FIG. 9A, as shown in FIG. 9C. When the photoresist film 6 is then separated. as shown in FIG. 9D, the rubbing direction in the region covered with the photoresist film 6 is the direction shown by the arrow A because the surface of the alignment film 3 is not subjected to second rubbing, while the rubbing direction in the region not covered with the photoresist film 6 is the direction shown by the arrow B because the surface of the alignment film 3 is subjected to second rubbing. In this way, two regions subjected to alignment processing in different directions are formed in each of the pixels. The substrates 1 and 7 processed as described above are combined, and a liquid crystal 8 is injected into the space therebetween to realize the in-pixel divided alignment structure in which liquid crystal molecules 9 are inclined in opposite directions in the same pixel, as shown in FIG. 9E.
However, the process for alignment in different directions in the divided regions of each fine pixel comprises rubbing, coating photoresist over the entire surface, pattering, rubbing, and separating the photoresist, and is thus complicated, thereby causing the problem of decreasing yield and increasing the production cost. Also this method requires two times of rubbing, and has the need to leave the second rubbing direction in the region where first rubbing and second rubbing are overlapped. There is also the problem of causing difficulties in appropriately controlling conditions of each time of rubbing processing.
Also a liquid crystal display device having the above in-pixel divided alignment structure causes disturbance (referred to as "disclination" hereinafter) in alignment of liquid crystal molecules in the vicinity of the boundary between two regions in each pixel having different alignment directions of liquid crystal, and thus has the possibility of leakage of light in this region. Therefore, a measure is required for making the leakage of light unnoticeable by providing a linear black mask on the opposite substrate side. However, this method causes the problem of decreasing the aperture ratio. Particularly, it is disadvantageous that such a black mask must be provided at the center of each pixel. Namely, this method cannot solve the problem of decreasing the aperture ratio even by employing the in-pixel divided alignment structure for widening the viewing angle.